Police in Bangladesh fired rubber bullets and tear gas on Sunday at thousands of Islamic activists demonstrating for the restoration of a religious pledge to the nation's constitution.

Scores of demonstrators and police officers were injured during anti-government clashes in the industrial towns of Fatullah and Kanchpur, outside the capital Dhaka, police and local media said.

The main opposition Bangladesh Nationalist Party (BNP) and various Islamist parties called for the nationwide strike from Sunday morning to demand the vow in "absolute faith in Allah" be reinstated as a pillar of the constitution.

The government last month made changes to bolster the secular character of the original 1972 constitution, although Islam was retained as the state religion.

In a wave of political unrest, the BNP and its allies have also enforced a recent series of strikes over changes to the electoral system that they say unfairly favour the incumbent government.

Much of Bangladesh has been shut down for four of the last eight days and on Sunday shops, businesses and schools were closed.

Fatullah, home to hundreds of factories making clothes for top global retailers, was hit by hours of fighting on Sunday as police battled with 600 protesters – mostly madrassa students – who threw bricks and blocked roads.